The invention relates to a method producing hollow articles of plastics material by blow molding according to German Patent 22,23,580, hereafter called "Main Application".
By the method of the Main Application a preform is expanded and cooled in a blow mold by means of a liquefied gas at low temperature. At the beginning of the working cycle of the machine the liquefied gas, by which nitrogen is principally meant, flows through the warm pipe and the warm blow mandrel thus being vaporized. The vaporized gas at low temperature serves to inflate the preform. In the further course of the admission phase the pipe and the blow mandrel are also decreasingly cooled, so that increasingly less liquefied gas is vaporized. A mixture of gas and liquid and finally pure liquid mist passes into the preform. The liquid mist evaporates through contact with the warm preform. The vaporized gas escapes during the flushing phase, in which liquid nitrogen is also simultaneously sprayed in.
The advantage of this method is that with a single medium at low temperature the hollow article is inflated and cooled in a single working cycle. The previous division into a separate blowing phase with a medium at ambient temperatures and a separate, later cooling phase with a medium at low temperature is eliminated, because the blow phase merges continuously into the cooling phase.
The working cycle of the machine is thus appreciably shortened and the capacity of the machine is thus increased. Only one medium has to be controlled. Particularly in the case of smaller hollow articles, for which the cycle times are correspondingly short, the additional advantage of a considerable simplification of the installation is achieved.
In the case of large hollow articles the specific consumption of nitrogen decreases. This may assume such proportions that more nitrogen is required for inflation than for cooling. Furthermore, it may occur that the heat content of the nitrogen pipe and of the blow mandrel will then no longer suffice to vaporize sufficient liquid nitrogen. It is therefore proposed in the Main Application that in such cases the blow mandrel or the nitrogen pipe should be provided with an additional heating system.
Although large hollow articles of plastics materials can be produced faultlessly in this manner, in some cases it is unsatisfactory to vaporize valuable liquid nitrogen solely for the purpose of providing sufficient gas to inflate the hollow articles.